IETF RFC 3573

scope:

The Layer 2 Tunneling Protocol (L2TP) defines a mechanism for
tunneling Point-to-Point Protocol (PPP) sessions. It is common for
these PPP sessions to be established using modems connected over
the public switched telephone network.

One of the standards governing modem operation defines
procedures that enable a client modem to put the call on hold and
later, reestablish the modem link with minimal delay and without
having to redial. While the modem call is on hold, the client phone
line can be used to place or receive other calls.

The L2TP base protocol does not provide any means to signal
these events from the L2TP Access Controller (LAC), where the modem
is physically connected, to the L2TP Network Server (LNS), where
the PPP session is handled.

This document describes a method to let the LNS know when a
client modem connected to a LAC has placed the call on hold.